In an electronic parts packaging line for fabricating package substrates in which electronic parts are mounted on a substrate, a screen printing apparatus for printing an electronic part joining paste on a substrate is connected to an upstream side of an electronic parts installation apparatus which installs electronic parts such as semiconductors on a substrate. As one of types of substrates on which electronic parts are packaged, there is conventionally known a cavity substrate in which electrode patterns are formed on both an upper surface of the substrate and bottom surfaces of recess portions which are opened to the upper surface of the substrate, and such cavity substrates have been in use in various equipment as a light, highly dense substrate (Patent Document 1). In a screen printing for printing an electronic part joining paste on a cavity substrate like this as a printing target, a screen mask is used which has a flat plate portion which is brought into contact with an upper surface of a substrate and fitting portions which are formed so as to project downwards from the flat plate portion to fit in the recess portions. By employing a three-dimensional screen mask like this, the paste can be printed both on the upper surface of the substrate and the bottom surfaces of the recess portions simultaneously.